In order to detect a vertical tear of a conveyor belt (cracks continuous in the belt longitudinal direction), a conveyor belt in which loop coils are embedded is known (see Japan Unexamined Patent Publication Nos. 2014-31241 and 2015-71493). In such a conveyor belt, a sensor arranged adjacent to the conveyor belt senses the induced current occurring in loop coils passing near the sensor. If a sharp object or the like to be conveyed pierces the conveyor belt and a vertical tear is generated, the loop coils are damaged, so no induced current is generated in the loop coils. In this case, the sensor does not detect the induced current even though the loop coils have passed through the vicinity. Thus, it is possible to determine whether a vertical tear has occurred in the conveyor belt depending on whether the induced current is detected by the sensor. If it is determined that a vertical tear has occurred on the basis of the detection of the sensor, the operation of the conveyor is stopped in order to prevent expansion of the vertical tear.
Local impacts and external forces are exerted on conveyor belts due to objects to be conveyed or the like to be loaded. Even if a vertical tear does not occur in the conveyor belt due to the impact or external force, the loop coils may be damaged. If loop coils are damaged in this way, the sensors do not detect the induced current even if the loop coils passing through the vicinity. Thus, if judged based on the detection of the sensor, it will be erroneously recognized that vertical tears occur in the conveyor belt.
Japan Unexamined Patent Publication No. 2015-71493 proposes loop coils comprising a metal part and a stretchable conductive material. In this proposal, the durability of the loop coils is improved by mitigating the impact or the like acting on the conveyor belt with the stretchable conductive material. However, in the case where a large impact or stress locally acts on the conveyor belt, it is difficult to prevent damage to the loop coils even if a part of the stretchable conductive material is provided. In addition, when a large impact or stress directly acts on the metal part, the mitigating effect by the stretchable conductive material can hardly be obtained and damage to loop coils cannot be prevented. Therefore, there is room for improvement in improving the durability of loop coils.